In California, it’s no ordinary Friday. Today marks the first April 20—a one-day stoners’ pride parade also known as 4/20—since the state legalized recreational marijuana.

The epicenter of celebration should arguably be this weekend’s High Times Cannabis Cup SoCal: a three-day festival and trade show in San Bernardino, where an estimated 20,000 people are expecting to enjoy performances by rappers that include Nas, Lil Wayne, and 2 Chainz; a slew of munchie-satisfying food trucks;...